The proposed research has three specific aims: (a) to investigate the relationship among cortisol concordance, behavioral synchrony of mother-infant pairs, and maternal attentiveness, (b) to explore the relationship between behavioral synchrony and cognitive development in infancy, and (c) to test the hypotheses on a population which is underrepresented in the research literature on attachment, specifically, a largely Hispanic population. A longitudinal design will be used, and data will be collected on primiparous mother-infant pairs at these infant ages: one week old, 3 months, 6 months, 12 months and 18 months. Salivary cortisol samples will be taken from the infant and the mother to determine whether or not each exhibits an adrenocortical response to a mildly stressful event (pre-experiment to post-experiment). Behavioral synchrony will be measured by observing specific behaviors of the mother and infant in 10-second intervals across a 10-minute interaction period. As a more distal measure of maternal sensitivity, mother's attentiveness to her infant will be recorded during time periods when her infant is participating in visual and auditory discrimination tasks. The infant will participate in experiments during four of the test sessions, to assess various aspects of cognitive development and their association to cortisol concordance, behavioral synchrony, and maternal attentiveness. It is hypothesized that infants or more sensitive mothers will score higher than infants of less sensitive mothers on measures of discrimination, memory, and object labeling, and that maternal sensitivity will amount of time spent in joint attention with her infant in a novel situation. The study will provide valuable information about the emotional, social, and intellectual health of infants in a diverse population.